EXPERIMENTAL INVESTIGATION OF MACHINING PARAMETER FOR WET & NEAR-DRY EDM FINISHING

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1 International Journal of Design and Manufacturing Technology (IJDMT), ISSN (Print), ISSN (Online) Volume 2, Number 1, May October (2011), pp IAEME, International Journal of Design and Manufacturing Technology (IJDMT), ISSN (Print), ISSN IJDMT I A E M E EXPERIMENTAL INVESTIGATION OF MACHINING PARAMETER FOR WET & NEAR-DRY EDM FINISHING ABSTRACT 1 Prakash R. Cheke, 2 Prof. D. S. Khedekar 1 Student, MGM s Jawaharlal Nehru Engineering College, Aurangabad, Maharashtra, India 2, Associate Professor, MGM s Jawaharlal Nehru Engineering College, Aurangabad, Maharashtra, India E mail: prcheke@rediffmail.com The correct selection of manufacturing conditions and technique is one of the most important aspects to take into consideration in the majority of manufacturing processes and, particularly, in processes related to Electrical Discharge Machining (EDM). This paper investigate wet and near-dry electrical discharge machining to achieve the high fine surface finish. In near-dry EDM liquid and air mixture delivered through a tabular electrode instead of only liquid dielectric used in wet EDM. L-9 orthogonal array is applied to investigate the effect of discharge current, pulse on time, gap voltage and pulse off time on surface finish in wet and near-dry EDM. Pulse on time and discharge current are identified as a key factors for improving the surface finish in wet and near-dry EDM. Finally a, comparison for performance of wet and near-dry EDM has been made. It was experimentally found that near-dry EDM exhibits the advantage of good machining stability thus result in smooth surface finish at low discharge energy. Keyword: Near-Dry electrical discharge machining, Wet electrical discharge machining 1. INTRODUCTION Electrical discharge machining (EDM) is a prevalent nontraditional precision machining method. The EDM machined surface is composed of discharge craters overlapping each other. To achieve a super surface finish, ultra low pulse energy1)-4) and powder mixed dielectric (PMD) EDM are two existing methods. Both methods have their limitations. Luo et al.1),2) demonstrated a super finished EDM surface with 0.04 µm Ra by reducing the pulse energy with a very low discharge duration of ti less than 0.2 µs. Egashira et al.3) applied the ultra low discharge energy, less than 3 J, to conduct machining with the assistance of 28

2 International Journal of Design and Manufacturing Technology (IJDMT), ISSN (Print), ISSN ultrasonic vibration. Okada et al.4) studied the radio-frequency plasma to produce a surface roughness of 0.15 µm Ra on aluminum. The disadvantage of the low pulse energy method is the low material removal rate (MRR) and long machining cycle due to the frequent abnormal discharges resulting from the narrow gap distance. The PMD EDM has been recognized as a more practical finishing process to generate very fine surface finish at relatively high MRR5),6). Near-dry EDM, first explored by Tanimura et al. 11), utilizes the liquid-gas mixture as the dielectric fluid. It is found to be beneficial for the finishing process with good machining stability12). Better surface finish was achieved in near-dry EDM than that of wet and dry EDM at the same energy level. It is hypothesized that the liquid particles dispersed in the gas medium has a similar mechanism to promote the finishing process as the additive powders in PMD EDM. The dielectric disposal of near-dry EDM is cleaner than that of PMD EDM. In addition, the EDM milling configuration uses a small tubular electrode to scan over the large working area. In this case, the stray capacitance, which increases with the overlapping area between the electrode and workpiece, is no longer a constraint on the machining area, as it did in conventional die-sinking EDM1). In this study, the near-dry EDM milling is investigated to understand the effect of dielectric fluid, electrode material and pulse energy level. Benefits and potentials of near-dry EDM milling as the finishing process are identified and discussed 2. PLANNING OF EXPERIMENT OHNS tool steel material having size 5 x x mm with hallow copper electrode 10 mm diameter were used. Separate work piece is used for each experiment. And the PS ZNC (die-sinking type) of EDM machine are used. Commercial grade EDM oil (specific gravity= 0.763, freezing point= 94 C) and air through a hallow tube was used as dielectric fluid with a pressure of 0.1 kgf/cm 2 and 0.5 kgf/cm 2 respectively. Circular shaped hallow copper tool with internal flushing of air was used to flush away the eroded materials from the sparking zone. In this machining time and duty cycle is kept constant is 7 min and For a four factor are tackled with a total number of 9 experiments were performed on wet sinking EDM for finishing condition and 9 experiments were performed on near-dry EDM for finishing condition. The surface finish measurement can be using contact type C3A Mahr Perthen Perthometer ( stylus radius of 5 µm ). 29

3 International Journal of Design and Manufacturing Technology (IJDMT), ISSN (Print), ISSN Table 1 Design Scheme of machining parameters and their levels Parameters Unit Levels Level-I Level-II Level-III Discharge current ( Ip ) Amp Pulse on time ( Ton ) µs Pluse off time ( Toff ) Position gap voltage ( Vg ) Volt RESULT AND DISCUSSION The analysis was made using the popular software specifically used for design of experiment applications known as MINITAB 16. Before any attempt is made to use this simple model as a predictor for the measures of performance, the possible of interactions. In the study, because of four factor three level L9 orthogonal array was used for design of experiment. Table 2 Experimental design matrix & results for Ra in wet and near-dry EDM. Exp. No. Ip Ton Toff Vg Ip Ton Toff Vg Ra for wet EDM( µm ) Ra for near-dry EDM( µm ) Influences on surface finish in wet EDM The response of signal to noise ratio for the factors is shown in Table 3 which is clearly indicates that the Ip has greatest influence on surface finish (SF), Vg has 30

4 International Journal of Design and Manufacturing Technology (IJDMT), ISSN (Print), ISSN the next greatest influence, followed by Toff and Ton. The delta values for Ip, Ton, Toff and Vg are 3.71, 1.24, 1.62 and 2.08 respectively, depicted in Table 3. Table 3. S/N Ratios for wet EDM EDM Level Ip Ton Toff Vg Delta Rank Table 4. S/N Ratios for near-dry Level Ip Ton Toff Vg Delta Rank For surface finish, case is Smaller is better, It is recommended that signal to noise(s/n) ratio is always maximum for optimal result. The response tables show that the S/N ratios were maximized when the Ip was 2, the Ton was, the Toff was 12 & Vg was 45. Examining the main effects plots for S/N ratio confirms these results. Main Effects Plot for SN ratios Data Means Ip Ton Main Effects Plot for SN ratios Data Means Ip Ton M e a n of SN r atio s Toff V g 1 M e a n o f S N r a t io s Toff Vg Signal-to-noise: Smaller is better Signal-to-noise: Smaller is better Fig. 1 Main effect plot for wet-edm Fig. 2 Main effect plot for near-dry EDM During the process of EDM, the influence of various machining parameter like Ip, Ton, Toff and Vg has significant effect on surface finish, as shown in main effect plot for S/N ratio of surface finish in Fig 1. Increasing in the discharge current from 2 to 4 A the surface finish decreases due to increase in discharge energy. 31

5 International Journal of Design and Manufacturing Technology (IJDMT), ISSN (Print), ISSN (Online) Volume 2, Number 1, May October (2011), IAEME 3.2 Influences on surface finish in near-dry EDM The response of signal to noise ratio for the factors is shown in Table 4 which is clearly indicates that the Ton has greatest influence on surface finish, Ip has the next greatest influence, followed by Toff and Vg. The delta values for Ip, Ton, Toff and Vg are 2.987, 3.084, and respectively, depicted in Table 4. For surface finish, case is Smaller is better, It is recommended that S/N ratio is always maximum for optimal result. The response tables show that the S/N ratios were minimized when the Ip was 2, the Ton was 100, the Toff was 11 & Vg was. Examining the main effects plots for S/N ratio confirms these results. During the process of EDM, the influence of various machining parameter like Ip, Ton, Toff and Vg has significant effect on surface finish, as shown in main effect plot for S/N ratio in Fig COMPARISON OF WET AND NEAR-DRY EDM Experiments were conducted for the same input parameters for w e t & n e a r - dry EDM. The results are shown in Figure 3. It can be seen that the SF in near-dry EDM is much better than that of the wet EDM for same parameters in each experiment in finishing. Chart for SF in wet & near-dry EDM finishing SF in wet EDM SF in near-dry EDM SF in µm Exp. No. Fig 3 Comparison of SF in wet EDM & near-dry EDM A comparison of process performance of wet EDM proven not beneficial for finishing because stable machining conditions are not established at low energy input thus result in a small discharge gap hence depth of cut increases where as in near-dry EDM proven beneficial for finishing operation because liquid phase is dispersed in gas medium is hypothesized to enhance the electric field and thus results in a large discharge gap distance and stable discharge at low energy input. The gases in the air prevent the electrolysis and yield better surface finish. 32

6 International Journal of Design and Manufacturing Technology (IJDMT), ISSN (Print), ISSN (Online) Volume 2, Number 1, May October (2011), IAEME 7. CONCLUSION The following conclusions are drawn for the effective machining in wet and near-dry EDM for finishing purpose. (1). Finding the result of surface finish is most influencing factor and then gap voltage(ip) and followed by pulse off time(toff) and pulse on time(ton) in wet EDM finishing and Ton is most influencing factor and then Ip and followed by Toff and Vg in near-dry EDM finishing (2). A comparison of process performance of wet EDM proven not beneficial for finishing because stable machining conditions are not established at low energy input thus result in a small discharge gap hence depth of cut increases where as in near-dry EDM proven beneficial for finishing operation. REFERENCES [1] Y.F. Luo and C.G. Chen Effect of a Pulsed Electromagnetic Field on the Surface Roughness in Super finishing EDM Precision Engineering, Vol. 12, No. 2 (1990) [2] Y.F. Luo, Z.Y. Zhang and C.Y. Yu Mirror Surface EDM by Electric Field Partially Induced Annals of CIRP, Vol. 37, No. 1 (1988) [3] K. Egashira, A. Matsugasako, H. Tsuchiya, and M. Miyazaki Electrical Discharge Machining with Ultralow Discharge Energy Precision Engineering, Vol. 30, No. 4 (2006) [4] A. Okada, J.A. McGeough, and D. MacMillan Machining Characterstics of EDM by Radio-Frequency Plasma Annals of CIRP, Vol. 55, No.1(2006) [5] H. Narumiya, N. Mohri, N. Saito, H. Ootake, Y. Tsunekawa, T. Takawashi and K. Kobayashi EDM by Powder Suspended Working Fluid Proceedings of 9th International Symposium on Electromachining (ISEM IX) (1989) 5 8. [6] N. Mohri, N. Satio and M. Higashi A New Process of Finish Machining of Free Surface by EDM Method Annals of CIRP, Vol. 40, No. 1 (1991) [7] Jia Tao, Albert J. Shih, and Jun Ni, Experimental Study of the Dry and Near-Dry Electrical Discharge Milling Processes (2008) Journal of Manufacturing Science and Engineering 130, , DOI: / [8] Jia Tao, Albert J. Shih, and Jun Ni, Near-dry EDM Mirror-Like Surface Finish Jan.(2008) International Journal of Electrical Machining, No. 13. [9] Masahiro Fujiki, Gap-Yong Kim, Jun Ni & Albert J. Shih, Gap control for near-dry EDM milling with lead angle International Journal of Machine Tools & Manufacture 51. [10] Grzegorz Skrabalak, Jerzy Kozak, Study on Dry Electrical Discharge Machining Proceedings of the World Congress on Engineering, June 2010 Vol-III. [11] T. Tanimura, K. Isuzugawa and I. Fujita Development of EDM in the Mist Proceedings of 9th International Symposium on Electromachining (ISEM IX) (1989) [12] J. Tao and A.J. Shih Dry and Near-Dry Electrical Discharge Milling Processes ISEM XV (2007)